Interpretive Summary: Recent studies have brought the use of genetically modified plants for pest management under increased public and scientific scrutiny. One critical issue about the use of genetically modified plants is that pollen from genetically modified corn (Bt corn) may have negative effects on non- target larvae (caterpillars) of species including monarch butterflies. Field observations were made in Maryland and Nebraska to determine if monarch butterfly larvae were present on milkweed leaves when corn pollen was shed. Milkweed plants in and near the cornfields contained the highest amount of corn pollen. The farther away the milkweeds were from the cornfield, the less amount of corn pollen was found on their leaves. In both Maryland and Nebraska, monarch activity was not seen until near the end of the study. These observations suggest that monarch activity does not occur at the same time as corn pollen release, and that the majority of milkweed leaves are safe for monarch larvae.

Technical Abstract:
Exposure risk to the monarch butterfly depends on the amount of Bt corn pollen that falls on milkweeds. To address this risk factor, studies were conducted in 1999 to quantify the pollen deposition on milkweed plants associated with cornfields at peak pollination. In addition, field observations were made to determine if monarch butterfly larvae were present on milkweeds at the time of pollen shed. Three areas with distinct topographical differences were selected, one in Nebraska and two in Maryland.